%% auteur : 
-2 2 setxrange
-2 2 setyrange
jpegmode
/A {-1 0} def
/cerc {O 1} def
/alpha 60 def
10 setfontsize
/m {alpha cerc cpoint} def
/M {A O m m perp orthoproj} def

/X {settvar
    t Cos t Sin dup mul sub
} def
/Y {settvar
    t Sin t Cos 1 add mul
} def

0  2 pi mul settrange

/Y' {settvar
    t Cos dup dup mul add t Sin dup mul sub
} def
/X' {settvar
    t Cos 2 mul 1 add t Sin mul neg
} def


continu

/dessin {
   1 dict begin
   1.5 setlinewidth
   gsave
      bleu
      {X} {Y} courbeparam
   grestore
   end

   /pas .075 def
   /alpha tmin def
   gris
   tmax tmin sub pas div round cvi {
      /alpha alpha pas add store
      alpha (X) (Y) dptangente 
      alpha X alpha Y perp
      droite
   } repeat
} def

dessin